Ingelman-Sundberg M, Rane A, Gustafasson J A
Biochemistry. 1975 Jan 28;14(2):429-37. doi: 10.1021/bi00673a033.
The substrate specificity of the steroid sulfate-hydroxylating activity in microsomes from human fetal liver has been investigated. Twelve different C18, C19, C21, and C27 steroid sulfates and the corresponding free steroids were used as substrates. The introduction of a sulfate group on the steroid substrate was found to have two principal effects. (1) The hydrophilic sulfate group directs the steroid molecule so that it only interacts with the active site of cytochrome P-450 with its non-sulfurylated, hydrophobic end. (2) The sulfate group interacts with the enzyme surface resulting in exposure of a slightly different part of the hydrophobic end of the substrate to the active site of cytochrome P-450 than when the same end of the free steroid is exposed to the active site of the enzyme. As a consequence of these two effects of the sulfate group, the "steroid sulfate pathway" of steroid hydroxylations generally differs considerably from the "free steroid pathway," both from a qualitative and a quantitative aspect. This difference was found to be most pronounced with estrogens: whereas estradiol was not hydroxylated by human fetal liver microsomal preparations, estradiol 3-sulfate was both 15-alpha- and 16-alpha-hydroxylated. Thus, for certain steroids, sulfurylation is a prerequisite for further metabolism by microsomal hydroxylase systems. These results indicate the presence in human fetal liver microsomes of a multipotent, highly unspecific, hydrophobic "bulk" of cytochrome P-450. The existence of this hydroxylase system which efficiently hydroxylates steroid sulfates is probably of great physiological importance as a detoxifying mechansim in the human fetus.
对人胎儿肝脏微粒体中甾体硫酸酯羟化活性的底物特异性进行了研究。使用了十二种不同的C18、C19、C21和C27甾体硫酸酯以及相应的游离甾体作为底物。发现在甾体底物上引入硫酸酯基团有两个主要作用。（1）亲水性硫酸酯基团引导甾体分子，使其仅通过其未硫酸化的疏水端与细胞色素P-450的活性位点相互作用。（2）硫酸酯基团与酶表面相互作用，导致底物疏水端与细胞色素P-450活性位点接触的部分与游离甾体相同端暴露于酶活性位点时略有不同。由于硫酸酯基团的这两种作用，甾体羟化的“甾体硫酸酯途径”在定性和定量方面通常与“游离甾体途径”有很大差异。发现这种差异在雌激素中最为明显：虽然人胎儿肝脏微粒体制剂不能使雌二醇羟化，但硫酸雌二醇3-硫酸酯可进行15-α-和16-α-羟化。因此，对于某些甾体，硫酸化是微粒体羟化酶系统进一步代谢的先决条件。这些结果表明人胎儿肝脏微粒体中存在一种多能、高度非特异性的疏水“整体”细胞色素P-450。这种能有效羟化甾体硫酸酯的羟化酶系统的存在可能作为人类胎儿的一种解毒机制具有重要的生理意义。
